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AGC is a feature and attenuation is a function. Automatic Gain control can work using an electronically controlled attenuation in the signal chain or by using variable gain amplifiers somewhere in the chain.
the golden rule of good signal processing is to control the gain as early in the signal chain as is possible. The best gain control is to put variable resistor right at the antenna terminal. The reason is very simple: the amplifiers down the chain have to handle weaker signals and weaker signals mean better fidelity. An audio AGC in a conventional superhet where most of the gain is in the IF stage will overload even on a moderate signals due to the distortions in IF amplifiers.
Picture this, there is the lowest level signal that the receiver can resolve, this is set by the bandwidth and the noise figure of the receiver. The highest signal that the receiver can resolve without distorting is determined by the IIP3 and the phase noise of the local oscillator. The range of signals between the minimum discernable signal and the loudest signal that the receiver can handle without noticeable distortion is your dynamic range.
Now, consider what happens if you switch on a 10 db attenuator between the antenna and the receiver. the MDS (minimum discernable signal) has to be 10 times more powerful and on the other hand, the signal level at which the distortion starts to show is also up by 10 db. In effect, to borrrow Rob, NC0B's words, the dynamic range is a moveable window that can be shifted up or down with attenuators.
Most receivers are too sensitive for HF bands. For instance, the ubitx is too sensitive for 40 meters, the atmospheric noise is 100 times more powerful than the internal noise (noise floor) of the receiver. This is wasted dynamic range. if we had thrown in a 20 db attenuator, we would hear exactly the same signals as before but our dynamic range would have been 100 times more. Dynamic range doesn't only apply when you are contesting or having a neighbour who transmits 1000 watts, it is a measure of how sweet the receiver sound is as well.
So, why is ubitx so needlessly sensitive? That is, because it is meant to work from 10 meters to 80 Meters. That sensitivity will be needed when the 15 meter opens up in a few years from now.
RF attenuation does remain the best place to control the gain, even for an AGC system. The challenge is to build a smoothly varying attenuator that doesn't need any active devices. Active devices in wide open receivers like the ubitx can severally compromise the receiver performance.
I am planning to do a video to explain who careful gain distribution is the key to a good radio design.
On Fri, Nov 20, 2020 at 9:38 AM TD <dlee@...> wrote:
> "what component is P1" ?????